Research On Fabrication And Mechanical Properties Of Warp-knitted Spacer Fabric Reinforced Syntactic Foam

As a rapid developing textile material in recent years,warp-knitted spacer fabric(WKSF)shows excellent compression,impact resistance and cushioning properties.It can effectively improve the interlaminar shear and the compression strength of composite in the thickness direction by adding WKSF into the matrix.Therefore,the WKSF is particularly suitable for acting as a "reinforcing skeleton" of various composite materials.Syntactic foams are materials formed by embedding hollow microballoons in matrix,the cell combined with wall structure of microballoons endow syntactic foam relatively high mechanical properties and lower density.By taking advantage of these properties,syntactic foams are being used in aerospace,marine,vehicles and architectural fields.Nevertheless,lower strength of traditional microballoons leads to poor mechanical performance of syntactic foams,which severely limits the further development of this material.Based on above reasons,the syntactic foam reinforced by warp-knitted spacer fabric(SFWKSF)was prepared in this research,and the interface combination status as well as mechanical properties,such as compression,flexural and impact properties have been studied.The purpose of this work is to provide theoretical guidance and experimental support for the optimization design of this new type composite.To better understand the effect of reinforced phases structures and parameter on the mechanical properties of SF-WKSF,four WKSFs with different structures and parameters were designed and produced,including two lapping movements of spacer-guide-bar,two surface structures of spacer fabrics and two diameters of spacer yarns.Finally,eight SF-WKSF samples from four WKSFs and three hollow glass microballoons were fabricated by hand lay-up method.Meanwhile,a traditional syntactic foam(NSF)was prepared as a comparative sample in this study.As a three-phase composite,the mechanical properties of SF-WKSF are strongly influenced by the microballoons distribution uniformity and interface combination status among fiber,particle and matrix in the composite.Hence,in this paper,the distribution of microballoons,and the interface bonding conditions between microballoons and matrix were analyzed by SEM micrographs of sample fracture surface.The results showed that the distribution of microballoons is relatively uniform,and microballoons are not aggregate near fiber and cluster in the matrix.Meanwhile,in order to investigate the interface bonding conditions between spacer yarn and syntactic foam matrix,multiple fiber/microballoons/epoxy composite samples were prepared by using three types of microballoons and four kinds of polyester fiber.And then,the classical microbond test was carried out on an Instron 5567 universal material testing system using a pair of tailor-made adjustable blades.According to the microbond test result,the interface bonding conditions between fiber and syntactic foam matrix is good,which illustrates that the sample preparation method in this research is reasonable.The results also demonstrate that the fiber diameter,microballoons volume fraction and wall thickness of microballoons have significant influence on the interfacial bond properties of the composite.The samples possessing smaller fiber diameter,lower microballoons contents and larger wall thickness microballoons show better interfacial bond properties.In view of the large data variability from microbond test,the finite element models based on the contact analysis of ANSYS was established to simulate the interfacial shear strength and stress distribution of fiber/microballoons/epoxy ternary composite.The results show that the force-displacement curves generated by the simulation model pullout tests showed good agreement with the forcedisplacement curves produced by the experiments.The relationships between the microbond test geometrical parameters and the interfacial shear strength from the simulation were consistent with the corresponding relationships from experiment.Meanwhile,the von Mises stress and contact friction stress distributions from simulation models can be used to explain the mechanism of debonding in microbond test.In order to investigate the compressive properties of SF-WKSF,a quasi-static compression test of SF-WKSF was carried out on the MTS810 universal testing machine.Based on the analysis of experimental data,it can be found that the typical stress-strain curve of SF-WKSF displays a twostage form(linear elastic region and plateau region),which is completely different from the threestage form of stress-strain curve for traditional syntactic foam.It showed that the compression properties of SF-WKSF were obviously controlled by structures and parameters of WKSF.The composites with larger lapping movements of spacer-guide-bar and denser surface layer structure exhibit higher compression modulus and yield strength.The sample with coarser spacer yarn(0.2mm)shows slightly higher yield strength compared with the sample containing thinner spacer yarn(0.16mm),however,the compression modulus values of these two composites are close.Moreover,different types and content of microballoons have important impact on the compression properties of specimens,the samples embedded with larger wall thickness microballoons and appropriately higher content of S60 HS microballoons display better compressive propertiesThe Mori-Tanaka method was used to predict the compression modulus of SF-WKSF.The results show that the model can effectively simulate the values and variation tendency of compression modulus for different samples.It should be noted that this model is especially suitable for predicting the compressive modulus of SF-WKSF with smaller wall thickness and moderate volume fraction of microballoons.Meanwhile,in order to predict the compressive strength of SFWKSF,two predictive models based on the fiber buckling theory and the transverse cracking theory,respectively,are presented in this paper.It can be found that the prediction results based on the transverse crack theory display poor accuracy,meanwhile,the prediction results based on the fiber buckling theory can precisely simulate the values and variation tendency of compressive strength for different SF-WKSF specimens.Considering the usage scenarios of SF-WKSF,three-point bending test was carried out to investigate the flexural properties of this new material.Moreover,the bending mechanism of SFWKSF was explained according to the least resistance path theory and the failure morphology of specimens.It can be found from the test results that in the case of similar density,the SF-WKSF displays superior properties than NSF with respect to the flexural strength and modulus.Meanwhile,the bending properties of SF-WKSF were obviously controlled by some structural parameters of WKSF.The composites with larger lapping movements of spacer-guide-bar and closer surface layer structure exhibit better performance on the anti-bending capacities.However,the effect of fineness of the spacer yarns on flexural properties is not significant.In addition,different types and content of microballoons have obvious influence on the bending responses of specimens,which can be improved by using microballoons of larger wall thickness and increasing content of S60 HS microballoons within an appropriate range.In order to investigate the dynamic mechanical properties of SF-WKSF,eight SF-WKSF samples and one traditional syntactic foam sample were tested on the Instron Dynatup 9250 HV drop weight impact tester with the velocities ranging from 1 m/s to 2 m/s.The low-velocity impact properties of specimens are characterized by peak impact force,major damage energy and absorbed energy.The results show that except the specimen embedded with B3 WKSF,all the SF-WKSF samples show superior impact resistance properties than NSF with respect to the peak impact force and major damage energy.And it also found that the composites showed better impact properties when constructed with larger lapping movements of spacer-guide-bar,coarser spacer yarn and closer surface layer structure.Furthermore,the composites with larger wall thickness of microballoons and appropriately higher content of S60 HS microballoons display higher peak impact force,major damage energy and lower absorbed energy.In the respect of finite element simulation,the ANSYS/LS-DYNA and LS-Pre Post were used to simulate the low-velocity impact properties of SF-WKSF.ANSYS/LS-DYNA was applied to establish the finite element model,define contact parameters,mesh,add constraints,set solution parameters,solve and implement post-processing.In addition,the LS-PrePost was used to implement post-processing and output the result data of finite element simulation.The finite element model consisted of four parts: the impact tup,matrix material,surface layer composite plate and spacer yarns.By contrast,the predict results of the deformation,peak impact force,major damage energy and the initial stage of force-time curve are in agreement with experimental results.